Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess, is a protozoan parasite that infects more than 480,000,000 people worldwide and causes 50,000 deaths yearly. Despite the clinical importance of this organism, little is known regarding the nature of protective immunity to amebic infection. In work supported by AI 30084, we studied the structure and function of a unique surface antigen of amebae, the serine rich E. histolytica protein, (SREHP), demonstrated that recombinant SREHP is a protective antigen in a rodent model of infection, and developed a severe combined immunodeficient (SCID) mouse model of amebic infection. The first objective of this proposal is to understand the nature of protective immunity to amebiasis using the SCID mouse model of amebic liver abscess. By selectively reconstituting SCID mice with defined lymphocyte populations, and through the administration and blockade of specific cytokines, we expect to delineate the components of protective immunity to amebic liver abscess, and any role the host immune response might play in exacerbation of disease. The second objective of this application is to develop an effective oral vaccine to prevent amebiasis, and to learn more about the nature of the mucosal immune responses induced by oral vaccination. We have developed two recombinant SREHP-based oral vaccines, one which utilizes the genetic fusion of SREHP to the cholera toxin B subunit, and a second recombinant construct, where SREHP is delivered to gut mucosal immune tissue by expression in an attenuated S. typhimurium strain. We will measure and compare the nature of the mucosal immune responses to SREHP and amebae induced by each of these vaccines, and assess their protective efficacy against E. histolylica colonization and disease in rodent and non--human primate models of internal amebiasis. The studies proposed should provide insights into the biology of the host response to amebiasis, vaccine-induced mucosal immune responses, and ultimately may provide a recombinant-SREHP based oral anti-amebic vaccine suitable for testing in man.